FSH and LH release during the first ovulation period of immature rats pretreated with PMS.

Abstract

Plasma and pituitary FSH and LH were measured by nadioimmunoassay at different times during the first ovulation period (from 1100 h on day 28 to 0800 h on day 29) of immature rats pretreated with 3 IU PMS on day 26. Gonadotropin changes were compared with those of normal 4-day cyclic rats during the peniovulatory period (from 1100 h on proestrus to 0800 h on estrus) to elucidate the mechanism responsible for the delay of the second estrus in these PMS-primed rats. A rapid increase of plasma FSH and LH occurred after the critical period in the immature rats as well as in the normal cyclic rats; the peak level of FSH was higher and that of LH lower in immature rats than in adults. Plasma FSH in the immature rats decreased after 2300 h on day 28, contrary to the adults in which elevated plasma FSH persisted until 0500 h of the morning of estrus. The LH surge in the adult rats was transitory showing a different pattern from FSH. In the immature rats the LH surge lasted 3 h longer than in the cyclic rats. Pituitary FSH and LH contents of cyclic rats rapidly decreased after the critical period. FSH was replenished during the night (from proestrus to estrus) despite persisting high plasma FSH levels. In contrast there was only a slight restoration of pituitary LH during this period. On the other hand, in immature rats pretreated with PMS, pituitary FSH and LH contents revealed the same pattern: after the critical period there was a gradual decrease until 0200 h on day 29, followed by a slight increase in the morning. These results indicate that the duration of the FSH surge during the first ovulation period in these PMS-pnimed immature rats may not sustain the threshold time necessary for the initiation of follicular maturation of the new set of follicles for the succeeding estrous cycle. The regulatory pattern of pituitary gonadotropin and/or the sensitivity of the pituitary to gonadotropin releasing factors in the immature rats have not yet differentiated into the regulatory pattern in the adult.